feat(updater): make pubkey optional in cli + bundler

[wovw]

This is the companion PR to tauri-apps/plugins-workspace#3114, addressing tauri-apps/plugins-workspace#2438.
Currently, the Tauri CLI strictly requires the updater pubkey to be hardcoded in tauri.conf.json so it can verify that the private key matches the public key during the build/bundling process. This prevents developers from securely fetching or providing the public key dynamically at runtime. This PR relaxes that build-time constraint.

Changes

• Changed pubkey to an Option<String> in UpdaterConfig and UpdaterSettings.
• Updated the bundling logic in tauri-cli/src/bundle.rs to handle an optional pubkey.
• If the pubkey is missing from the config, the CLI now skips the public/private key verification step and instead emits a compile-time warning reminding the developer to ensure it is set at runtime.
• The strict requirement for the pubkey to be present before performing an update is now handled at runtime in fix(updater): make pubkey optional in tauri config, check if set at runtime (fixes #2438) plugins-workspace#3114
• cargo test passes